In the course of this investigation, we intend to examine the functional significance of sperm motility for penetration of the egg envelopes. This study will be performed under highly-controlled culture conditions utilizing a new approach in which the degree of sperm motility is precisely regulated by means of a substance, referred to as 'sperm motility factor', that is present in spermatozoa and many other body tissues. In addition, we will provide information that will indicate whether or not the sperm motility factor has a functional role in mammalian sperm motility, by correlating its presence in spermatozoa with their motility characteristics. Finally, we will identify the chemical nature of the motility factor by means of standard purification and analytical techniques.